1. Field of the Invention
The present invention relates to a radiation detector. In particular, although not exclusively, the present invention relates to a radiation detector for use in a lithographic apparatus. The radiation detector may be used, for example, to detect a wavelength of radiation in the EUV or DUV parts of the electromagnetic spectrum. The radiation, and/or more specifically a wavelength of radiation, may be used to apply a pattern to a substrate using the lithographic apparatus. The radiation detector may be an image alignment sensor.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
UV radiation is commonly used to provide pattern features in a layer of resist. In order to be able to project ever smaller structures onto substrates, it has been proposed to use extreme ultraviolet radiation (EUV) having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm or 6-7 nm.
Extreme ultraviolet radiation (amongst, for example, other wavelengths of radiation) may be produced using, for example, a plasma. The plasma may be created for example by directing a laser at particles of a suitable material (e.g., tin), by directing a laser at a stream of a suitable gas or vapor such as Xe gas or Li vapor, or by creating an electrical discharge. The resulting plasma emits extreme ultraviolet radiation (or beyond EUV radiation), which is collected using a collector such as a mirrored grazing incidence collector, which receives the extreme ultraviolet radiation and focuses the radiation into a beam.
Practical EUV Sources, such those which generate EUV radiation using a plasma, do not only emit desired ‘in-band’ EUV radiation, but also undesirable ‘out-of-band’ radiation. This out-of-band radiation is most notably in the deep ultra violet (DUV) radiation range (100-400 nm) and the visible radiation range (400 nm-700 nm). Moreover, in the case of some EUV sources, for example laser produced plasma EUV sources, the radiation from the laser, usually at 10.6 μm, presents a significant amount of out-of-band radiation.
The energy of photons constituting a beam of UV radiation can be high, and high enough to damage surfaces or objects exposed to the UV radiation. The photon energy, and thus the potential for damage, increases as the wavelength of radiation is shortened, for example to wavelengths in the DUV and EUV parts of the electromagnetic spectrum.
UV radiation, and in particular EUV radiation, is known to cause out-gassing of objects exposed to that EUV radiation. This out-gassing can result in the generation of contamination, which may be deposited on any one or more of a number of surfaces within the lithographic apparatus. This can lead to degradation in the optical performance (e.g., reflectivity) of those one of more surfaces.
In a lithographic apparatus, it is often necessary to be able to detect one or more wavelengths of radiation used by the lithographic apparatus to apply patterns to a substrate. Generally, an intensity of that radiation will be measured, and the measured intensity may be used, for example, to perform alignment of a beam of radiation (e.g., providing on comprising an image) that is used by the lithographic apparatus. Commonly, photodiodes are used as radiation detectors in a lithographic apparatus. However, photodiodes have a number of disadvantages associated with their use.
The exposure of photodiodes to radiation can cause degradation of the photodiode. For example, when the radiation comprises high energy photons (for example, 90 eV photons of a EUV beam of radiation) the photons can cause degradation and damage of the photodiode. Another disadvantage is associated with the deposition of contamination referred to above. For example, out-gassing of or from a surface can lead to the deposition of contamination on a surface of the photodiode. For example, carbon may be deposited on one or more surfaces of the photodiode. Such deposition can lead to a reduction or failure in the ability of the photodiode to detect radiation of one, more or all wavelengths. Currently, no cleaning process is known for photodiodes, meaning that contaminated photodiodes would need to be replaced, which could be expensive. A yet further disadvantage associated with the use of photodiodes is that photodiodes may respond to (e.g., detect) the out-of-band radiation referred to above, and this may provide a false indication of the level or intensity of desired wavelengths of radiation constituting a beam of radiation.